It is known to form a cross-sectional profile of a tubular member by a hydro-forming process in which a fluid filled tubular blank is placed within a cavity of a die and then the die is closed so that the tubular blank is captured within the die. Fluid pressure is then increased inside the tubular member to expand the blank outwardly against the cavity of the die to provide a tubular product having a die formed cross-sectional profile.
During tube hydroforming, punches of various shapes are used to create necessary holes in the tube. The single body punches currently used, are adequate enough for creating holes, but, with some limitations. During piercing the only back support of the tube wall is the forming pressure. As an example, the low forming pressure of 7000 psi, used to form the majority of the tubular side rails, of the automotive frame, is not enough to hold the wall around the periphery of the hole from collapsing inward during piercing. A hole with such configuration makes it difficult to attach reinforcing parts, or any other parts, on the inside and/or in the immediate vicinity of the hole.
Methods of piercing holes using a double action punch have been designed, built, and proven. The first segment of the punch pierces the center part of the hole first, forming a rectangular hole. At the same time, it splits the slug in the center and folds it to the sides. The second segment of the punch starts to activate forward at the moment when the first segment reaches half stroke. The second segment of the punch is designed to shear two sides of the hole on a desired angle, preferably between 30 or 45 degrees, therefore applying more side force during cutting and minimizing the collapse of the hole edges. From this position on, both segments of the punch will travel together until the end of the predetermined stroke.
As the number of hydroformed tube applications grows, the need for complex piercing also increases. Tubes are not only designed with more holes, but also with larger, more sophisticated shapes, which in turn need more elaborate punch designs for piercing. The need to reduce secondary operations as well as improve quality is becoming more important. However, the methods currently used are not designed to pierce large holes in parts formed with low pressure. In order to reduce cost, the hydroforming industry is constantly searching for ways to reduce the need for laser or plasma cutting holes as a secondary operation and more ways for piercing all the necessary holes in the parts, during the hydroforming process.
The method currently used in piercing is conventional. Upon completion of hydroforming, the part is in intimate contact with the wall of the die. The hydroforming fluid is at the forming pressure, approximately 7,000 psi. A punch is attached to a hydraulic actuated cylinder. When the hydraulic cylinder is extended, the fluid pressure will support the tube around the circumference of the punch, allowing the metal to shear. The punch will shear a slug, which will fall as a loose piece and lay inside the formed tube, or in other situations, the tip of the punch will be provided with a 45 degree chamfer, allowing the slug to be retained on the edge of the hole.
As a result, it is desirable to provide a double action punch assembly to pierce holes in a tube during the hydroforming process. It is also desirable to provide a double action punch that can withstand extremely high forces. It is further desirable to provide a punch assembly that is more compact than a hydraulic cylinder of equal power. Therefore, there is a need in the art to provide a new double action punch assembly for a hydroforming die that meets these desires.